Mark asked: What is the best way to clean flagstone? I have a light colored flagstone patio and walkways which get very dirty from the elements and mold (shaded areas). I have a power washer and have used strictly water, but it needs something better.
The 
best way to clean flagstone regularly is to use a pressure washer with plain water. However, sometimes dirt and stains require more than water for removal. Use the steps below to remove stains and tough dirt from your flagstone.
General Cleaning & Stain Removal
You Will Need:
- Dish liquid
- Bucket
- Water
- Scrub brush
- Ammonia
- Kerosene
Steps to Clean the Flagstone:
- Put 1 tablespoon of dish liquid in a bucket with a gallon of water. Mix it well so that the soap forms suds.
- Dip a scrub brush in the solution and scoop some suds off the surface of the water onto the scrub brush.
- Scrub the flagstone clean.
- If the stains remain, mix some ammonia into the bucket. If you don’t have ammonia, ¼ cup of kerosene can be used instead.
- Put on gloves, then scrub the stone with the brush and soapy water.
- Rinse the area thoroughly with water when the stains are gone.
Removing Mold
You Will Need:
- Boiling water
- Scrub brush
- White vinegar
- Water
- UV light
Steps to Remove the Mold:
- Boil water in a tea kettle or pan, then pour the boiling water onto the area. Be careful not to let any water splash on you. Scrub the area with a scrub brush after pouring the water. Be careful not to scratch the flagstone while scrubbing.
- If you can’t use boiling water, another option is to pour white vinegar over the area. Let the vinegar sit on the stone for about a minute, then rinse the area thoroughly with water and scrub with a scrub brush. Do not let the vinegar sit on the flagstone for very long as the acid can damage the stone. The vinegar is used to kill the mold spores to prevent it from returning.
- A UV light can be used instead of boiling water or vinegar to kill the spores. Put the UV bulb in a desk lamp that has a movable neck and point the light at the mossy area for at least 30 minutes. If the area is large, you may need to use multiple lights or move the light several times to ensure the entire area is treated.
Additional Tips and Ideas
- Always use caution when working with ammonia. Wear gloves and work in a well-ventilated area. For more ammonia safety information, see Wikipedia.
- Avoid using any scouring cleansers as they can damage the surface of the stone.
- If you need to both remove mold and clean the area, start by killing the mold using the methods in the Remove the Mold section, then follow up with the method for General Cleaning and Stain Removal.
Sources
- 1001 Hints & Tips by Reader’s Digest
- Haley’s Cleaning Hints by Graham and Rosemary Haley
- 99 Cent Solutions by Reader’s Digest
I think it might be wise to mention the fact that muriatic acid is an extremely dangerous chemical, and that anyone using it might want to have the necessary equipment needed (rubber boots, heavy duty rubber gloves, vinyl-coated cover-alls, full-face protection with a respirator, etc.). Your article gives absolutely no indication of this.
I use Oxyclean (oxygen bleach) to clean my bluestone patio. I give it a quick rinse/scrub/rinse to get the majority of the dirt off, then I scrub it w/ a mixture of 1 scoop per gallon and let it sit for 4-5 minutes; I quick scrub again, and rinse. Oxygen bleach works well and it is safe for the plants/grass.
Also, if you use a sealer, it can’t be one that truly seals the stone… it needs to breathe. If it can’t breathe, the sealer will blotch up and you’ll have to remove it, which is a terrible experience.